The present invention relates to novel fluid catalytic cracking catalysts comprising microspheres containing more than about 40% by weight Y-faujasite zeolite and having exceptionally high activity and other desirable characteristics, methods for making such catalysts and the use of such catalysts for cracking petroleum feedstocks, particularly those containing large amounts of contaminant metals.
Since the 1960's, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, called microspheres, containing both an active zeolitic component and a non-zeolitic component. Frequently, the non-zeolitic component is referred to as the matrix for the zeolitic component of the catalyst. The non-zeolitic component is known to perform a number of important functions, relating to both the catalytic and physical properties of the catalyst. Oblad described those functions as follows:
"The matrix is said to act as a sink for sodium in the sieve thus adding stability to the zeolite particles in the matrix catalyst. The matrix serves the additional function of: diluting the zeolite; stabilizing it towards heat and steam and mechanical attrition; providing high porosity so that the zeolite can be used to its maximum capacity and regeneration can be made easy; and finally it provides the bulk properties that are important for heat transfer during regeneration and cracking and heat storage in large-scale catalytic cracking." A. G. Oblad, Molecular Sieve Cracking Catalysts, The Oil And Gas Journal, 70, 84 (March 27, 1972). PA1 (a) The surface area and the volume of pores having diameters in the range of 20-100.ANG. were determined by conventional nitrogen adsorption and desorption techniques, respectively, using a Micromeritics.RTM. Digisorb 2500 Automatic Multi-Gas Surface Area and Pore Volume Analyzer. Before being tested for surface area and volume of pores having diameters in the range of 20-100.ANG., the microspheres were first pretreated by heating them, under vacuum, at about 480.degree. F. for 16 hours. PA1 (b) The volume of pores having diameters in the range of 600-20,000.ANG. was determined by a conventional mercury intrusion porosimetry technique using a scanning mercury porosimeter manufactured by Quantachrome Corp. The relationship between pore diameter and intrusion pressure was calculated using the Washburn equation and assuming a contact angle of 140.degree. and a surface tension of 484 ergs/cm.sup.2. Before being tested for volume of pores having diameters in the range of 600-20,000.ANG., the microspheres were pretreated by heating them, in air, to about 660.degree. F. for one hour and then cooling them in a dessicator. The mercury intrusion porosimetry technique described above, including the pretreatment of the microspheres, was also used to determine the volumes of pores having diameters in the ranges of 35-20,000.ANG. and 100-600.ANG. referred to in this application. PA1 (c) The bulk density of the 200/700 mesh fraction of the microspheres was calculated using the procedure described in the publication entitled "Engelhard Attrition Index" referred to above. In particular, the bulk density was calculated by dividing the weight of the "original sample" by the volume of that sample (0.661 cc.). The microspheres used to prepare the "sample volume" did not require any treatment for removal of electrostatic charge and were equilibrated at 30-70% relative humidity.
In prior art fluid catalytic cracking catalysts, the active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in-situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves tp provide microspheres containing both zeolitic and non-zeolitic components.
It has long been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, it must have good selectivity towards producing products that are desired and not producing products that are not desired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
Two products that are particularly undesirable in commercial catalytic cracking processes are coke and hydrogen. Even small increases in the yields of these products relative to the yield of gasoline can cause significant practical problems. For example, increases in the amount of coke produced can cause undesirable increases in the heat that is generated by burning off the coke during the highly exothermic regeneration of the catalyst. In addition, in commercial refineries, expensive compressors are used to handle high volume gases, such as hydrogen. Increases in the volume of hydrogen produced, therefore, can add substantially to the capital expense of the refinery.
A significant limitation of zeolite fluid catalytic cracking catalysts is the tendency of such catalysts (a) to be deactivated rapidly in the presence of metal contaminants often found in petroleum feedstocks, particularly vanadium, and (b) to function as a support for contaminant metals, particularly nickel and vanadium, which produce excessive amounts of dehydrogenation reaction products, i.e., coke and hydrogen. Because of these limitations and increased concern about the world's diminishing supply of petroleum, particularly petroleum with only small amounts of metal contaminants, in recent years substantial efforts have been made to develop improved zeolitic cracking catalysts useful for cracking petroleum feedstocks containing relatively large amounts of vanadium and nickel.
A phenomenon that has inhibited the development of zeolitic catalysts having improved activity, stability, and selectivity characteristics is that frequently the improvement in one of those characteristics is accompanied by adverse consequences to one or more of the other characteristics or by other adverse consequences which make the process for making the catalyst economically unattractive. A practical effect of this phenomenon is that generally compromises must be made regarding conflicting catalytic and physical characteristics of the catalyst. For example, U.S. Pat. No. 4,326,993 states that "[i]n general, for a given type of catalyst, attrition resistance increases with . . . decreasing pore volume." The '993 patent goes on to state, however, that "while low pore volumes are desirable, too low a pore volume can lead to selectivity losses due to diffusional restrictions. Desirable values are therefore a compromise" ('993 patent, col. 4, lines 54-59).
To the best of our knowledge, there has been no commercial fluid catalytic cracking catalyst comprising microspheres containing more than about 35% by weight Y-faujasite and less than about 65% by weight of a nonzeolitic component. The reason for this is that it is difficult to increase the Y-faujasite content of microspheres of fluid catalytic cracking catalysts past about 35%, or decrease the non-zeolitic component of the microspheres below about 65%, by a commercially practicable process without adversely affecting one or more of the microsphere's activity, selectivity, hydrothermal stability and attrition resistance characteristics to such an extent as to make the microspheres commercially unattractive.